Applications of spectrometers, especially in applied spectrometry for determining chemical composition and deformulation, chemical properties, structural integrity and chemical verification against counterfeit or mishap, are of great interest to the pharmaceutical, forensics, biotechnology, food and agriculture, mining, mineralogy, gemology, petroleum exploration, medical diagnostics, electronics and other industries. For food and agriculture, spectrometry has been applied for determining the composition and ripeness of food as well as for detecting contamination due to harmful chemical agents and pathogens such as infectious bacteria. Nutritional information of food ingredients and food in solution may be determined using methods such as Raman spectrometry, which uses a spectrometer to measure the shifts in light (visible and/or infrared) from a monochromatic excitation, typically from a laser, to other frequencies. In all applications, the wavelength or wavenumber, the inverse of wavelength, are important. In some cases, for example for Raman spectrometry, the difference between the excitation spectral line wavenumber and the measured spectral line wavenumbers are important.
FIG. 1 is a diagram that illustrates functional blocks and processing for a conventional spectrometer. Sometimes spectrometers are also called spectrographs or spectroscopes, and the terms are used interchangeably in this description. Spectrometers, spectrographs, and spectroscopes produce spectrograms, which are visual representations of spectral frequencies. Sometimes an entire system is described as a spectrometer, with a spectrograph or spectroscope described as a component of the spectrometer system.
Conventional spectrometer technologies include an internal or external light source 2, an optional specimen for determining absorption, transmission or re-emission 4, a spectroscope 6 that produces a spectrogram, a detector 8, processing for removing background signal and noise 10, and optionally further normalization 12 for the cases of absorption and transmission measurement. FIG. 2 is a block diagram illustrating a conventional Raman spectrometer. As shown in 24 of FIG. 2, the conventional spectroscope 6 of FIG. 1 typically has optional front end optics 26 and 28, a monochromator 30 (an optical dispersion spectral separator) such as an optical grating, prism or similar mechanism, optional second optics (sometimes combined with the detector as in a camera 32), and an optically isolating housing which may or may not include the entire detector.
Spectrometry, which in general is the application of spectrometers to study objects, typically requires a controlled light source, commonly a laser or broad band source. However, the spectrometer is often a separate device and does not include the light source. The detector within the spectrometer is generally sensitive across a broad band of radiation frequencies. In some cases, tristimulus detectors have been used, for example Charge Coupled Device (CCD) cameras with optical red (R), green (G) and blue (B) filters, but only for determining a single intensity estimate along the position of the spectrogram. This intensity estimate is taken directly from a color image, which is comprised of red, green and blue (RGB) primaries. Since the intensity is associated only with spatial position along the spectrogram, the resulting spectral resolution is limited to the resolution of the optics. The resolution of the optics is typically primarily determined by the width of the slit opening where the light enters the spectrometer.
For example, the Rspec Explorer is a relatively inexpensive commercially available spectrometer, available from fieldtestedsystems.com. Its USB camera is housed in a black box which also includes a diffraction grating and lens. A separate pair of adjustable black foamboard panels are supplied to provide an optical slit, which may be a few feet or further away from the black box. This external slit determines the limit of the optical resolution and therefore also the spectral resolution. A narrow slit improves resolution, but limits the light level for the spectrometer. Thus there is the classic trade-off between spectral resolution and signal to noise ratio. The USB camera is an RGB based CCD camera that captures the conventional image of the camera, with the diffraction grating image super-imposed on the right side. Software that runs on a PC takes this image and creates magnitudes from the RGB image of the spectrogram.
The determination of measured wavelengths or wave numbers from the typical spectrometer are generally determined by spatial location, in turn determined by design, requiring strict adherence to particular alignment of all components in the optical path, and usually further refined through calibration. The difficulty of alignment has been mitigated in many cases by using lenses and filters in two directions, with beam splitters and other optical components that tend to be lossy, that sometimes causes marginal signal-to-noise ratios.
In the Rspec Explorer example, the user must align the peak of the slit from the direct camera image to a reference line (graticule) shown in a window of the corresponding software application on the computer. The alignment is typically somewhat tedious and imprecise as the peak is often too broad due to the slit being too wide. If instead the slit is narrow enough for precise alignment, the resulting spectrum magnitude is typically near or below the noise floor of the camera, or the reference peak is so high that it is beyond the dynamic range of the camera, resulting in clipping. This clipping means limiting the peak to the maximum camera digital code for amplitude in the respective channel(s). So, as with other spectrometers, the wavelength resolution of the Rspec Explorer is no better than the optical resolution determined by the combined point spread function of a slit, the resolution of the CCD, and the intermediate optics. And, as with other spectrometers, because of the trade-off between light intensity and resolution due to slit width, the camera's CCD dynamic range, determined by sensitivity and noise floor, also factors into the determination of the optical resolution and thus the spectral resolution.
Another type of existing spectrometer is one that uses a simple very low cost spectrogram and a web cam detector. Many of the key performance issues with this prior art is summarized in Kong Man Seng, “Trace gas measurement using a web cam spectrometer,” March 2011, City University of Hong Kong, Department of Physics and Materials Science. The “Discussion” of section 5, page 33-40, discusses the typical issues with slits, alignment, optics, noise and similar issues that cause limited optical resolution and dynamic range. This same section includes some of the typical methods for mitigating these issues, the vast majority of which depend on improving optical resolution directly, improving alignment mechanically and increasing light source power along with heat and other energy management required to prevent damage to components due to the increased radiation.
In general, typical methods for improving spectrometer accuracy have been to increase optical resolution and precision, increase precision of overall mechanical and optical alignment, increase light power, and to increase detector dynamic range. Each of these increases cost with ever diminishing returns of improvement. Detector dynamic range is typically improved by reducing the noise floor and allowing for integrating steady state or repeated signals over time. The most common detector uses CCD technology. For highest dynamic range, the CCD is cryogenically cooled to mitigate one form of noise, while other forms of noise are still present. The noise types can be rebalanced by custom CCD design, thus improving cryogenic performance. The resulting detector system may be orders of magnitude more expensive than one based on consumer camera technology.
Many alternative designs attempt to mitigate issues associated with the loss of light through filters, narrow slits, etc., typically by adding more expensive optical components, light sources and the like.
Further, depending on the specific optical arrangement, often the wavelength as a function of distance along the spectrogram primary (frequency) axis is non-linear and follows a cosine function. Cosine correction is optionally included, often complicating the design.
These improvements in accuracy add significant expense. Many of the methods to improve accuracy, especially in combination, also cause the instruments to be large, bulky and prevent portability, or require additional significant expense to reduce size. Also, when integration is required to compensate for weak signals due to small slit size, the stability of the light source becomes critical, thus increasing cost, size and complexity of the light source. For the extra electronics and light power, the power supply required becomes significantly larger and more expensive.
Embodiments of the invention address these and other limitations of the prior art.